Phylum: Charophyta   Family: Desmidiaceae

Cosmarium exiguum W. Archer 1864

The Wests (1908) specifically mention this species as being 'Common in the Outer Hebrides'. Recent sampling would suggest it's not so common these days. Found in acidic bogs, especially with Sphagnum.

References: 
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3.
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.